Should it cost more to build a foundation for a new addition when only 2 walls are needed to meet 2 existing?

We don't really need more basement at present, but fear the repercussions of putting a new addition over a crawlspace or pilings, only to later wish we had the basement....but...2 local contractors rudely gave the same quick guess of $15K. One of them also provided this: "concrete $13 a ft, block $8 each, and you can figure it out yourself". So, I tried that and came up with only $4K, add maybe $2k for footings, and that's only 6k---so? As I told them: we want to add a 15x10 addition and this will fill in the empty corner of an L-shaped house, so we only need 2 new walls. I can understand the issue of mating a new full height basement to the existing 6 ft. basement, but even they dismissed that as some simple benching. It would seem they simply figured $100/sq. ft. for the 150 sq.ft. addition. With the details the one guy gave me I umed he meant $13/ sq. ft. concrete (i.e. floor) and for blocks I used 8ft as the height, and this only gave the forementioned 4k. How to calculate?